1. Field of the Invention
The present invention relates to an olefin-polymerizing catalyst capable of forming in one polymerization system two homopolyolefins, each of said homopolyolefins being an isotactic polyolefin in which the disorder of monomer units alignment is in respective specified range. The present invention further relates to a process for producing an olefin polymer using said catalyst and a polypropylene produced by said process which is useful for production of a biaxially oriented film. More particularly, the present invention relates to a process for producing an olefin polymer excellent in stiffness and stretchability and successfully usable for formation of films, fibers, blow-molded articles, extrusion-molded articles, etc., an olefin-polymerizing catalyst used for production of such a polymer, a polypropylene for use in the production of a biaxially oriented film which can give a biaxially oriented film excellent in transparency and stiffness while retaining a particularly excellent stretchability, and a biaxially oriented film produced from said polypropylene.
2. Description of Related Art
Biaxially oriented films are extensively used as packaging materials because of their excellence in transparency, gloss, stiffness and water vapor barrier property. With the aim of improving the film properties, such as stiffness, of biaxially oriented film, highly crystalline homopolymers of propylene obtained with a high-stereospecificity catalyst have conventionally been used. However, such highly crystalline homopolymers of propylene have a fault that they are inferior in stretchability and apt to make troubles such as film breaking in the course of stretching. Thus, a variety of methods have so far been proposed for improving the stretchability of highly crystalline polypropylene. As one example of such proposals, copolymerization of propylene with a small amount of ethylene is known. Concretely speaking, JP-B-46-11027 has proposed a process for producing a polypropylene for use in production of films which comprises polymerizing propylene together with a very small amount of ethylene in the presence of a coordination catalyst and a hydrocarbon, hydrogen chloride or chlorinated hydrocarbon solvent so that one mole of the monomer unit in the resulting polymer becomes containing 0.01 mole or less of ethylene unit. Further, in JP-B-64-6211, there is proposed a process for producing a polypropylene improved in stretchability which comprises feeding propylene together with a small amount of ethylene into polymerization system using a catalyst consisting of an organoaluminum compound and titanium trichloride prepared by reducing titanium tetrachloride with an organoaluminum compound and further activating the reduced product by a treatment using a complex-forming agent, a treatment using an organoaluminum compound, a treatment using titanium tetrachloride or a combination thereof so that the resulting polymer comes to have an ethylene content of 0.1-1.0% by weight. Further, in JP-B-3-4371, there is proposed a process for producing a biaxially oriented polypropylene film excellent in transparency, stiffness and impact resistance by using a polypropylene of which ethylene content is 0.1-2% by mole and isotacticity is in a specified range. Any of these processes, however, have been unsatisfactory as a method for obtaining a polypropylene for a biaxially oriented film simultaneously having an excellent transparency and a high stiffness while retaining a good stretchability.